The bears start to move in October, when longer nights chill Hudson Bay and snow starts to fly. Along the stony western shore, they head north over the salt marsh toward Cape Churchill. Hunting season is about to begin, after a four-month fast since the annual ice breakup in July. Almost all summer the bears have been in "walking hibernation," sleeping in dens and occasionally wandering through a vast boggy lowland called Wapusk National Park, living mainly off their fat reserves.
But soon shoreline ice will form. By walking north, the bears know they will ﬁnd it and their staple prey—ringed seals—faster. In November when the ice usually thickens enough to walk on, hundreds of male bears and nonpregnant females roam far from shore, scanning and snifﬁng breathing holes of unwary seals. About 200 pregnant females remain behind, for Wapusk offers them excellent nurseries.